A personal computer, running a classic single-user single-tasking operating system, usually runs an X server as a dedicated application. When the X server is running, no other applications run. All the X clients are running on a server system elsewhere. The personal computer is connected to the server via a serial line or some type of network connection such as ethernet. If it is talking over ethernet, the personal computer identifies itself to the other system using its IP address and its ethernet address. An unauthorized user can change the IP address, and sometimes the ethernet address, to masquerade as another system.